The present invention relates to fuel injectors suitably used for injecting fuel to an engine, for instance, an automobile engine.
Generally, fuel injectors used for automobile engines include a tubular casing having an axial fuel passage and made of magnetic metal. A valve seat is disposed at one end of the fuel passage and has a fuel outlet. A core cylinder is disposed within the fuel passage in axially spaced relation to the valve seat. A valve element is axially moveably disposed within the fuel passage. An electromagnetic actuator is provided for forcing the valve element to an open position upon being energized. In the open position, the valve element is out of contact with the valve seat to allow fluid to be injected through the fuel outlet.
Japanese Patent Application First Publication No. 11-6467 discloses such an electromagnetically operated fuel injector. The fuel injector of this related art also includes a casing, a core cylinder axially opposed to the valve element with an axial air gap, and a non-magnetic joint disposed between the casing and the core cylinder. When the electromagnetic actuator is energized, a magnetic field extends to the core cylinder and the valve element via the axial air gap, so that the valve element is attracted by the core cylinder and moved to the open position. The non-magnetic joint suppresses a short-cut of the magnetic field which will be caused between the casing and the core cylinder. If the short-cut is caused, the magnetic field will form a closed magnetic circuit without passing through the axial air gap between the core cylinder and the valve element. This will cause reduction of the magnetic force acting on the valve element and the core cylinder.
Japanese Patent Application First Publication No. 2000-8990 also discloses an electromagnetically operated fuel injector of the same type as described above. The fuel injector includes a casing formed by a metal pipe, and an annular non-magnetic portion disposed in an axial-middle position of the casing. Upon energizing the electromagnetic actuator, the annular non-magnetic portion prevents the short-cut of the magnetic field. The annular non-magnetic portion is formed by subjecting the axial-middle portion of the metal pipe to heat treatment, for instance, induction heating.
Japanese Patent Application First Publication No. 2001-27169 discloses such an electromagnetically operated fuel injector of the same type as described above. The fuel injector includes a tubular casing having a fuel outlet at one axial end portion thereof, a resin cover covering an opposite axial end portion of the casing, a seal disposed on near the one axial end portion of the casing, and a resin protector for protecting the one axial end portion of the casing and the seal. Upon manufacturing the fuel injector, the parts such as a valve seat, a valve element, a core cylinder and an electromagnetic actuator are mounted to the casing, and the resin cover is formed by injection molding. An axial air gap (a valve lift amount) between the valve element and the core cylinder is adjusted using a tool. After the adjustment work, the resin protector previously molded is mounted onto the one axial end portion of the casing together with the seal.